High-efficiency radio-frequency (RF) amplifiers are finding increased use in communication base station applications. These high-efficiency RF amplifiers are desired because of the lower system size and cost achieved by the need for less cooling capability and because of the reduced energy needed to power these systems. Conventional high efficiency amplifiers (e.g. tuned class-AB) have theoretical efficiencies that approach 78% or, in some cases such as class-F, 100%. This high efficiency operation is achieved by terminating the active transistor of the amplifier at the harmonic frequencies of the frequency of amplifier operation. For example, an ideal tuned class-clAB amplifier may require all harmonics to be terminated in a short circuit. An ideal class-F amplifier may require terminating the even harmonics (i.e. 2nd, 4th, etc.) of the amplifier signal in a short circuit and the odd harmonics (i.e. 3rd, 5th, etc.) of the amplifier signal in an open circuit.
However, these conventional high efficiency RF amplifiers can be difficult to realize using packaged RF devices with matching circuitry realized external to the packaged RF devices, because the harmonic termination circuitry, especially the ideal open and short circuit terminations, may be difficult to realize using practical circuit elements. This often means that only a very narrow range of tuning conditions will provide high efficiency operation for RF amplifiers created with packaged devices. This narrow range of optimum tuning conditions introduces un-acceptable product variation and yield loss. In addition, the harmonic terminations may consume large areas of printed circuit board (PCB) in the amplifier system and thus compete with the goals of smaller area devices. Thus, amplifier devices with reduced sensitivity to external matching conditions are desired.